drm/gpusvm: Introduce drm_gpusvm_find_vma_start() function

drm/gpusvm: Introduce drm_gpusvm_find_vma_start() function

The drm_gpusvm_find_vma_start() function is used to determine the starting
address of a CPU VMA within a specified user range. If the range does not
contain any VMA, the function returns ULONG_MAX.

v2
- Rename function as drm_gpusvm_find_vma_start() (Matthew Brost)
- mmget/mmput

v3
- s/mmget/mmget_not_zero/

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250513040228.470682-13-himal.prasad.ghimiray@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>